1. Field of the Invention
The present invention relates to an injection molding process which enables a plastic article of a greater extruded area to be molded under a relatively low clamping force.
2. Prior Art
In injection molding, a thermoplastic molding material is plasticized in an appropriate heating cylinder, then forced by plunger action through one or more orifices into mold cavities where the material solidifies to the desired shape. In such injection molding, the pressure of the material within the cavities has been conventionally controlled as depicted in FIG. 1. Specifically, a molten resin is first injected at a gradually increasing pressure, as indicated at 100. Then, when each cavity has been filled, there will be a pressure boost, as at 102, since an injection pressure is additionally exerted on the mold as a hydrostatic pressure. With this pressure boost, the molten resin is filled up in each cavity with a sufficient filling density to thereby prevent shrinkage and warpage of the molded article from occurring. Subsequently, the pressure of the molten resin will be decreased stepwise, as indicated at 104 and 106. In the first half as at 104, a solidified layer formed in the surface portion of the molten resin is pressed against the mold, so that the shrinkage and warpage are prevented from occurring in the outer peripheral portion of the molded article. In the latter half as at 106, the shrinkage and warpage are further prevented.
To confine the material properly in the mold cavity during the application of the injection pressure, an adequate clamping force is maintained against the mold. The clamping force has been conventionally determined so as to be not smaller than a force F defined by the following equation: EQU F=P.times.A
where P is a maximum pressure of the resin within the cavity during molding operation and A is an extruded area or cross-section of the molded article. If the clamping force is smaller than the force F, a mold gap will be so enlarged that flash is caused to occur on the molded article. Specifically, in the conventional process described above, there is a pressure boost when the cavity has been filled with the molten resin, and the clamping force has to be determined in view of this pressure boost. Consequently, the extruded area of the article to be molded is limited substantially, resulting in low productivity.